Labels

Monday, October 21, 2013

Instantly Increase ASP.NET Scalability - Thru ThreadPool

Instantly Increase ASP.NET Scalability - Thru ThreadPool

Each time a request for a .NET resource (.aspx, page, .ascx control, etc.) comes in a thread is grabbed from the available worker thread pool in the asp.net worker process (aspnet_wp.exe on IIS 5.x, w3wp.exe on IIS 6/7) and is assigned to a request. That thread is not released back into the thread pool until the final page has been rendered to the client and the request is complete.

Inside the ASP.Net Worker Process there are Two Thread Pools. The Worker Thread Pool handles all incoming requests and the I/O Thread pool handles the I/O (accessing the file system, web services and databases, etc.).

But how many threads are there in these thread pools? I had assumed that the number of threads would vary from machine to machine – that ASP.NET and IIS would carefully balance the number of available threads against available hardware, but that is simply not the case.

ASP.Net installs with a fixed, default number of threads to play with: The CLR for the 1.x Framework sets these defaults at just 20 worker threads and 20 I/O thread per CPU.

Now this can be increased by modifying the machine.config, but if you are not aware of this, then 20 threads is all you’re playing with. If you have multiple sites sharing the same worker process, then they are all sharing this same thread pool.

So long as the number of concurrent requests does not exceed the number of threads available in the pool, all is well.

But when you are building enterprise level applications the thread pool can become exhausted under heavy load, and remember by default heavy load is more than just 20 simultaneous requests.

When this happens, new requests are entered into the request queue (and the users making the requests watch an hour glass spin).

ASP.NET will allow the request queue to grow only so big before it starts to reject requests at which point it starts returning Error 503, Service Unavailable.

To resolve contention problems, you can tune the following parameters in the Machine.config file to best fit your situation:

maxWorkerThreads

minWorkerThreads

maxIoThreads

minFreeThreads

minLocalRequestFreeThreads

maxconnection

executionTimeout

To successfully resolve these problems, take the following actions:

Limit the number of ASP.NET requests that can execute at the same time to approximately 12 per CPU.

Permit Web service callbacks to freely use threads in the ThreadPool.

Select an appropriate value for the maxconnections parameter. Base your selection on the number of IP addresses and AppDomains that are used.

Note The recommendation to limit the number of ASP.NET requests to 12 per CPU is a little arbitrary.

However, this limit has proved to work well for most applications.

maxWorkerThreads and maxIoThreads

ASP.NET uses the following two configuration settings to limit the maximum number of worker threads and completion threads that are used:

<processModel maxWorkerThreads="20" maxIoThreads="20">

The maxWorkerThreads parameter and the maxIoThreads parameter are implicitly multiplied by the number of CPUs.

For example, if you have two processors, the maximum number of worker threads is the following: 2*maxWorkerThreads

minFreeThreads and minLocalRequestFreeThreads

ASP.NET also contains the following configuration settings that determine how many worker threads and completion port threads must be available to start a remote request or a local request:

<httpRuntime minFreeThreads="8" minLocalRequestFreeThreads="8">

If there are not sufficient threads available, the request is queued until sufficient threads are free to make the request.

Therefore, ASP.NET will not execute more than the following number of requests at the same time: (maxWorkerThreads*number of CPUs)-minFreeThreads

Note The minFreeThreads parameter and the minLocalRequestFreeThreads parameter are not implicitly multiplied by the number of CPUs.

minWorkerThreads

As of ASP.NET 1.0 Service Pack 3 and ASP.NET 1.1, ASP.NET also contains the following configuration setting that determines how many worker threads may be made available immediately to service a remote request.

<processModel minWorkerThreads="1">

Threads that are controlled by this setting can be created at a much faster rate than worker threads that are created from the CLR's default "thread-tuning" capabilities. This setting enables ASP.NET to service requests that may be suddenly filling the ASP.NET request queue due to a slow-down on a back end server, a sudden burst of requests from the client end, or something similar that would cause a sudden rise in the number of requests in the queue. The default value for the minWorkerThreads parameter is 1. We recommend that you set the value for the minWorkerThreads parameter to the following value.

minWorkerThreads = maxWorkerThreads / 2

By default, the minWorkerThreads parameter is not present in either the Web.config file or the Machine.config file. This setting is implicitly multiplied by the number of CPUs.

maxconnection

The maxconnection parameter determines how many connections can be made to a specific IP address. The parameter appears as follows:

<connectionManagement>

<add address="*" maxconnection="2">

<add address="http://65.53.32.230" maxconnection="12">

</connectionManagement>

If the application's code references the application by hostname instead of IP address, the parameter should appear as follows:

<connectionManagement>

<add address="*" maxconnection="2">

<add address="http://hostname" maxconnection="12">

</connectionManagement>

Finally, if the application is hosted on a port other than 80, the parameter has to include the non-standard port in the URI, similar to the following:

<connectionManagement>

<add address="*" maxconnection="2">

<add address="http://hostname:8080" maxconnection="12">

</connectionManagement>

The settings for the parameters that are discussed earlier in this article are all at the process level. However, the maxconnection parameter setting applies to the AppDomain level. By default, because this setting applies to the AppDomain level, you can create a maximum of two connections to a specific IP address from each AppDomain in your process.

executionTimeout

ASP.NET uses the following configuration setting to limit the request execution time:

<httpRuntime executionTimeout="90"/>

You can also set this limit by using the Server.ScriptTimeout property.

Note If you increase the value of the executionTimeout parameter, you may also have to modify the processModel responseDeadlockInterval parameter setting.

Recommendations

The settings that are recommended in this section may not work for all applications. However, the following additional information may help you to make the appropriate adjustments. If you are making one Web service call to a single IP address from each ASPX page, Microsoft recommends that you use the following configuration settings:

Set the values of the maxWorkerThreads parameter and the maxIoThreads parameter to 100.

Set the value of the maxconnection parameter to 12*N (where N is the number of CPUs that you have).

Set the values of the minFreeThreads parameter to 88*N and the minLocalRequestFreeThreads parameter to76*N.

Set the value of minWorkerThreads to 50. Remember, minWorkerThreads is not in the configuration file by default. You must add it.

Some of these recommendations involve a simple formula that involves the number of CPUs on a server. The variable that represents the number of CPUs in the formulas is N. For these settings, if you have hyperthreading enabled, you must use the number of logical CPUs instead of the number of physical CPUs. For example, if you have a four-processor server with hyperthreading enabled, then the value of N in the formulas will be 8 instead of 4.

Note When you use this configuration, you can execute a maximum of 12 ASP.NET requests per CPU at the same time because 100-88=12. Therefore, at least 88*N worker threads and 88*N completion port threads are available for other uses (such as for the Web service callbacks).

For example, you have a server with four processors and hyperthreading enabled. Based on these formulas, you would use the following values for the configuration settings that are mentioned in this article.

Also, when you use this configuration, 12 connections are available per CPU per IP address for each AppDomain. Therefore, in the following scenario, very little contention occurs when requests are waiting for connections, and the ThreadPool is not exhausted:

The web hosts only one application (AppDomain).

Each request for an ASPX page makes one Web service request.

All requests are to the same IP address.

However, when you use this configuration, scenarios that involve one of the following will probably use too many connections:

Requests are to multiple IP addresses.

Requests are redirected (302 status code).

Requests require authentication.

Requests are made from multiple AppDomains.

In these scenarios, it is a good idea to use a lower value for the maxconnection parameter and higher values for the minFreeThreads parameter and the minLocalRequestFreeThreads parameter.